Trapping and manipulating ions is widely used in analytical techniques such as mass spectrometry (MS). Ion traps are also used for other applications such as quantum computing. Trapped ions can be used for accumulating a population of ions to be injected into an ion mobility drift cell to perform ion mobility spectrometry (IMS) to separate, identify, or distinguish ions or charged particles. IMS can be employed in a variety of applications such as separating structural isomers and resolving conformational features of charged chemical compounds, macromolecules, and essentially any charged particles. IMS may also be employed to augment mass spectroscopy in a broad range of applications, including metabolomics, glycomics, and proteomics, as well as for a broad range of applications involving essentially any compound that can be effectively ionized.
For example, when performing IMS in a conventional drift tube, a sample composed of ions having different mobilities can be injected into a first end of an enclosed cell containing a carrier gas, also referred to as a buffer gas. In the cell, the ions can move from the first end of the cell to a second end of the cell under the influence of one or more applied electric fields. The ions can be subsequently detected at the second end of the cell as a function of time. The sample ions can achieve a maximum, constant velocity (i.e., a terminal velocity) arising from the net effects of acceleration due to the applied electric fields and deceleration due to collisions with the buffer gas molecules. The terminal velocity of the ions increases with the magnitude of the electric field and is proportional to their respective mobilities, which are related to ion characteristics such as mass, size, shape, and charge. Ions that differ in one or more of these characteristics will exhibit different mobilities when moving through a given buffer gas under a given electric field and, therefore, will achieve different terminal velocities. As a result, each ion exhibits a characteristic time for travel from the first end of the cell to the second end of the cell. By measuring this characteristic travel time for ions within a sample, the ions can be distinguished or identified.
There are a number of IMS formats used for chemical and biochemical analysis, including constant field drift tube ion mobility spectrometry (DT-IMS), high field asymmetric ion mobility spectrometry (FA-IMS), differential mobility analysis (DMA), trapped ion mobility spectrometry (TIMS), and traveling wave ion mobility spectrometry (TW-IMS). These formats vary in the manner by which the electric field is applied to separate the ions within the IMS cell or device. However, in addition to separating ions, IMS devices ideally also confine ions within the device as the ions move through the device to prevent the ions from colliding with the surfaces of the device itself and causing loss of ions.
Ion traps, on the other hand, manipulate ions based on their mass to charge ratio. Ions react to electric field oscillation in radio frequency (RF) by executing a simple harmonic motion between electrodes on which the RF fields are applied. In this way, they remain in dynamic equilibrium and can be effectively trapped, manipulated, and interacted with by other ions, neutrals, photons, etc. This kind of ion confinement or ion trapping is possible at vacuum conditions (e.g., pressure less than 0.1 Torr). Using design variants, like stacked ring ion guides with opposite polarity of RF on adjacent electrodes, the pressure up to which ions can be trapped or confined within a volume can be increased to 50 Torr in devices such as ion funnels and ion funnel traps. However, ion trapping at much higher pressures is not possible using RF fields as the collisions of ions with background neutrals prevents them from executing simple harmonic motion critical to ion confinement.
For similar reasons as discussed above, conventional IMS devices can only efficiently confine ions at low pressures, typically no more than about 10 Torr in most devices or potentially up to about 50 Torr in some designs. As such, many conventional IMS devices are designed for operation in low pressure or near vacuum conditions using well understood methods involving pseudopotentials generated using RF electric fields. The combination of low pressure operation (e.g., the use of a confining chamber and an associated pumping system) and an RF power supply needed to create ion confinement fields greatly increases the size, weight, and cost of operating such devices and limits the applications for which they can be used. Additionally, the low pressure operation of IMS devices for confinement can lead to loss of ions during transport from an atmospheric pressure ion source to the vacuum stage in which the IMS device operates. Also, other applications could be greatly simplified by the ability to trap ions within a volume at higher pressures than is currently possible. Specifically, it would be highly advantageous to be able to confine and manipulate ions at atmospheric pressure.
Accordingly, there is a need for ion manipulation devices that can operate effectively at atmospheric pressure and more generally at all pressures (or at conditions with a low ratio of electric field strength to ion density, which depends on pressure).